Russian Olga Puchkova overcame Venus Williams on Friday to secure a berth in the final at the inaugural WTA Brasil Tennis Cup.

The unseeded Puchkova fought back to stun the fifth-seeded former world No. 1 American Williams 4-6, 6-4, 7-5 in 2 hours, 31 minutes in their first-ever meeting.

The Russian popped six aces, but also committed 11 double faults in the surprise victory. Puchkova broke Williams six times, while the seven-time Grand Slam champion settled for five breaks en route to defeat.

Puchkova's opponent in Saturday's final will be Romanian fellow 25-year-old Monica Niculescu, who also advanced via upset on Day 6. Niculescu ousted seventh-seeded 19-year-old Frenchwoman Kristina Mladenovic 6-0, 6-2 in 63 minutes on the hardcourts at Federacao Catarinense de Tenis.

The Bucharest native Niculescu, like Puchkova, is seeking her first-ever WTA title. She was a runner-up in Luxembourg in both 2011 and last year.

The Moscow native Puchkova is in search of her first WTA title in her third final. She hasn't advanced this far on tour since finishing as a runner-up on two occasions in 2006.

The 75th-ranked Niculescu and No. 109 Puchkova will meet for the first time on Saturday.

This week's champion will pocket $40,000.